Posted
by
Soulskill
on Wednesday July 30, 2014 @09:43AM
from the galactic-atkins dept.

schwit1 writes: New research by astronomers suggests that the Milky Way is about half as massive as previously estimated. It was thought to be roughly the same mass as Andromeda, weighing in at approximately 1.26 x 10^12 solar masses (PDF). This new research indicates its mass is around half the mass of Andromeda. "Galaxies in the Local Group are bound together by their collective gravity. As a result, while most galaxies, including those on the outskirts of the Local Group, are moving farther apart due to expansion, the galaxies in the Local Group are moving closer together because of gravity. For the first time, researchers were able to combine the available information about gravity and expansion to complete precise calculations of the masses of both the Milky Way and Andromeda. ... Andromeda had twice as much mass as the Milky Way, and in both galaxies 90 percent of the mass was made up of dark matter."

It's the inflation in play here, just like in the economy, the units of measurement are being stretched now, so the same mass measured in different units gives a nominally different answer

A better example would be the sizing of women's clothes. What is now a size 4 used to be a size 6 a few years ago but because women are getting fatter, the sizes had to change to make people feel good about themselves.*

* Men don't have to worry as much because their sizes are measured in inches.

* Men don't have to worry as much because their sizes are measured in inches.

Well, sort of.

Although more common in women's apparel, vanity sizing occurs in men's clothing as well. For example, men's pants are traditionally marked with two numbers, "waist" (waist circumference) and "inseam" (distance from the crotch to the hem of the pant). While the nominal inseam is fairly accurate, the nominal size may be smaller than the actual length by more than an inch in U.S. sizes. In 2010, Abram Sauer of Esquire measured several pairs of dress pants with a nominal waist size of 36 at different U.S. retailers and found that actual measurements ranged from 37 to 41 inches.[7] The phenomenon has also been noticed in the United Kingdom, where a 2011 study found misleading labels on more than half of checked items of clothing. In that study, worst offenders understated waist circumferences by 1.5 to 2 inches. London-based market analyst Mintel say that the number of men reporting varying waistlines from store to store doubled between 2005 and 2011.[8]

Men have been lying about a couple of inches in the pants forever too:)

Well thanks for that. I was actually wondering recently why it was so hard to shop for pants. I have a hard size anyway, as I am built for a much smaller inseam than my waist (or rest of my torso) would seem to indicate. In fact, I would say if you look at my torso vs legs, I have the torso of someone several inches taller than me, and the legs of someone an inch or two shorter.

Looking back, I think this is why my childhood doctor was always suggesting my weight should be unreasonably low based on her heigh

Well thanks for that. I was actually wondering recently why it was so hard to shop for pants. I have a hard size anyway, as I am built for a much smaller inseam than my waist (or rest of my torso) would seem to indicate. In fact, I would say if you look at my torso vs legs, I have the torso of someone several inches taller than me, and the legs of someone an inch or two shorter.

My stepson was that way as a teenager. His mother had to buy jeans that fit his waist, then have them altered to remove the extra six inches of length from the legs. And that was with buying the shortest inseam available in the waist size.

It AIN'T the men, its the stupid as hell clothing industry thinking any guy besides metrosexuals give a rat's ass about that. I don't know how many guys I've talked to the past few years that are ROYALLY PISSED that they can't just go into a store and grab a pair of size whatever and know it'll fit. We don't care if you call it a 37 or an ummagumma just STICK TO IT so we can just walk in and buy a pair of damned pants without having to try shit on like the girls...is that REALLY so much to ask for?

As for TFA? The correct answer is "we don't know shit, it'll probably change, but for now the number pulled out our collective asses is". hell they can't even make the math work without "dark" this and "dark" that which should automatically be replaced with a little stick figure shrugging its shoulders. I think the only thing we can truly say for certain is how things work in this one little teeny tiny itsy bitsy area, anything else? Its just wild guesses with a LOT of question marks in there.

Spot on. I have a pair of 32" jeans that would fall down if I wasn't wearing a belt, and a pair of 36" trousers that I need to squeeze into. I don't need to go into a changing room to see how these look on me, I'd just as happily take them straight from the shelf to the cashier if I could rely on the sizing.

lHow can they possibly tell how much of the matter is "Dark"? I can get the idea of what they're doing - using the relative speeds of each local galaxy to determine the masses contained within each, but how could they possible determine how much mass in each galaxy wouldn't be seen by using light within the bounds of the visible spectrum?

They measure the rotation curve - the orbital velocity at various distances from the galaxy's centre - and use those points to calculate the mass binding the orbiting stars. That give the total of dark + visible matter. The mass of normal, stellar matter is estimated from the star counts and knowledge of stars in our own galaxy.

Yes it would mean there is half as much dark matter in the milky way as we previously would have thought but then there's half as much normal matter as well so you get the same ratio. Of course this has no impact on the total dark matter budget of the universe so it doesn't really have any bearing on our understanding of dark matter.

All known stable forms of matter take only four forms: The lightest charged lepton, the electron, the lightest baryon, the proton, the nearly massless neutrinos, and atomic nuclei built from bound states of protons and neutrons. All but the neutrinos interact with matter in well-known ways that we can definitely measure (they are, or contain, electrically charged particles and therefore emit and scatter photons, which are relatively easily picked up). The neutrinos only undergo weak force interactions, but modern detectors can still pick them up. Their known number density and the limits on their mass mean they can't account for the observed gravitational binding of galaxies.

Any normal matter would interact with the light from objects behind it. This is the origin of effects such as the Lyman Forest which reveals the distribution of cold hydrogen in the flight path of distant light... The bottom line is, baryonic matter in the quantities implied simply has nowhere to hide. It's just too easy to detect ten times a galaxy's mass of matter sprinkled among the galaxy's visible part.

Hence the hypothesis of a particle which has mass but no electric charge, color charges or weak hypercharge. It would cast a gravitational shadow but otherwise be virtually undetectable since it does not undergo any interactions we can make individual particle measurements of. Hence the 'dark' in dark matter. One of the few alternatives to a dark particle is a universe suffused with low-mass black holes, but the lack of either microlensing events or gravitational waves emitted by their scattering off each other is difficult to explain. There are a few other places in GR that you can insert hypothetical terms without making it blow up in the face of observations - in fact Einstein's original cosmological constant very effectively explains accelerating expansion. It's also possible that GR isn't the correct theory of geometrodynamics, and the effect of higher-order curvature terms or such is not zero.

GR and QFT are fundamentally incompatible (GR is classical, QFT is quantum), so while there absolutely must be new physics out there, the question is where the new physics lays and what form it takes (and can we ever reach the energy levels to directly investigate it). The belief among physicists is that the correct theory should be the simplest one which fully explains observed phenomenon - Hence why, for example, GR as currently postulated does not involve any higher order curvatures - and the standard cosmological constant / cold dark matter framework does a remarkable job of explaining the evolution of the universe to its present state with remarkably few parameters.

Very lucid. Thanks for this. One of the things I've wondered is why it's not usually mentioned that "GR [may not be] the correct theory of geometrodynamics." I mean, why dark matter? Why not, "we don't understand gravity yet"? Or, we don't understand all the possible forces of attraction well enough? What about the Casimir Effect, for instance? What happens if that's somehow additive at cosmic scales? What if we've missed something? (Wouldn't be the first time.) It's probably very obvious that I am NOT any

The electric force is 10^36 times greater than gravity. Most humans, including physicists cannot really visualize a number with 36 zeros behind it. Because charges can either attract or repel, the electric forces in the universe ALMOST but not quite in some instances cancel each other. It is the ignoring of this tiny imbalance which is not being accounted for in the motion of the galaxies that causes astrophysicists to suggest the existence of dark matter, dark e

Because "we don't understand gravity yet" contains no data and makes no predictions (ie, it may well be true but it is unscientific). I think of dark matter as a list detailing exactly where and how much we don't understand gravity or cosmological particle physics. When someone wants to test a new theory of gravity, they will know where to check for discrepancies with GR by looking at where dark matter is; alternately, when someone wants to test a new theory for cosmological particle physics, they can test

The Casimir Effect is very short-ranged. It appears to be based on there being room for forming virtual particles or not (some actual physicist please correct me if I'm wrong).

AFAICT, we don't completely understand gravity, but that's not a scientifically useful statement. If somebody has suggestions for changing General Relativity that match current observations, that would be useful. Apparently, it's hard to do that, and nobody's really succeeded.

but how could they possible determine how much mass in each galaxy wouldn't be seen by using light within the bounds of the visible spectrum?

Such "dark matter" would show up on Xrays [harvard.edu] infrared [caltech.edu] or radio [nrao.edu], so that's not a problem. If, however, the "dark matter" does not interact with electromagnetism, but only with gravity and the weak force, (which would be an extremely odd, and frankly, a not very believable aspect of cosmology) things would get a bit tricky.

If, however, the "dark matter" does not interact with electromagnetism, but only with gravity and the weak force, (which would be an extremely odd, and frankly, a not very believable aspect of cosmology) things would get a bit tricky.

That is EXACTLY what most of the dark matter is suspected to be and that is what makes it tricky.

Always reminds me of the neutrino, first postulated to balance out some equations and found some 30 years later. I'm sure back then on the slashdot of the day you had people doubting that such a small non-interacting particle could exist.

They estimate the amount of dark matter from the rotational speed of the galaxy. Basically, a rotating body is in equilibrium between the centripetal pseudo-force that wants it to spread out into an infinitely wide disk, and the binding force (gravity in this case) that wants it to be a perfect sphere. When you look at the amount of visible matter and attempt predict its shape from its rotational speed and mass, you realise the galaxy is the wrong shape. There must be an extra force pulling the stars togeth

lHow can they possibly tell how much of the matter is "Dark"? I can get the idea of what they're doing - using the relative speeds of each local galaxy to determine the masses contained within each, but how could they possible determine how much mass in each galaxy wouldn't be seen by using light within the bounds of the visible spectrum?

You can see the light. So you do this:
1: Measure the mass of the galaxy.
2: Add up all the mass from the stuff you can see.
Subtract (2) from (1).

They've been mapping Dark Matter based on gravitational lensing. "Nothing" doesn't cause gravitational lensing, so we know something is there, and whatever it is, there is almost 10x more of it than what we can see in the entire EM spectrum from radio to gamma.

Well, to be fair, most of the mass is *not* in the stars, but the dark matter. It might be a reasonable inference that twice the stars would also mean twice the dark matter, but that might not necissarily be true.

So then really what they are confirming is that inference; that matter and dark matter are in a 1/9 ratio, and that if our galaxy has x stars and Andromeda has 2x stars, then Andromeda will also have 2y dark matter mass to our y matter mass.

Starting from the Earth getting kicked out from the center of the universe to the present hypothesis that visible matter is just a tiny fraction of all the stuff in the universe, having the mass of the Milky Way reduced is just another step in what Carl Sagan called The Great Demotions [google.com]. Hopefully by now humanity is getting used to it.

If they can be that wrong about something so fundamental, then how can they possibly claim to understand things or be right now?

I read an article recently about scientists saying the speed of light is not constant. Has their new variable speed of light calculation been plugged into all these other cosmology equations? Maybe this dark matter fudge factor would disappear and we would stop being wrong by the 1/2 the mass of a galaxy.

If they can be that wrong about something so fundamental, then how can they possibly claim to understand things or be right now?

It's not like they discovered that Andromeda is actually a 20-foot wide disco ball with funhouse mirrors making it look bigger than it really is. When you're talking about a branch of science that typically works in orders of magnitude, a factor of 2 is a pretty minor change.

Using Newton's equations and constant gravity on a particle-in-a-vaccum to calculate that I'd be in freefall for 1.8 seconds if I were climbing and fell at the end of a 25 foot runout is "wrong" because I'm not a particle, I'm not in a vaccum, gravity isn't position independent, and one really ought to be using GR instead of Newton. But 1.8 seconds is close enough for the purposes of knowing how long until the rope starts to catch me.

But maybe its harder to measure MilkyWay than to measure the mass of other galaxies. Still, they cannot directly measure the mass of anything out there. So they are implying the mass by looking at the light coming from them and from neighboring objects. I would rather look forward to them being proven wrong about their assumptions because we would learn more. Maybe they might learn something that could help us out here on Earth.

So I prefer to look at theories which challenge the accepted science in

We can look at a whole bunch of galaxies from a distance, and see the whole galaxy (except for the dark matter) somehow or another. There's precisely one galaxy that we can see from the inside, and we don't have a good global view of it. It's easier to categorize things we can observe in a similar way.

The article says that most of the galaxies are moving apart, but the Local Group is moving closer. Why would the local group be different than the other galaxies? Are there other groups of galaxies that are seeing the same effect, or is the Local Group an anomaly?

The article says that most of the galaxies are moving apart, but the Local Group is moving closer. Why would the local group be different than the other galaxies? Are there other groups of galaxies that are seeing the same effect, or is the Local Group an anomaly?

The galaxies in the local group are close enough together to be a gravitationally bound system, and are therefore "decoupled" from the expansion. This is true of any cluster of galaxies, and there are many, many examples of such systems in the universe.

It's the same reason your body doesn't get bigger as the universe expands: the binding forces holding it together are stronger than the (tiny) force pulling it apart due to cosmological expansion.

I'd assume that all galaxy groups are Gravitationally bound, and when looking at the group you're in, the galaxies would appear to be closing, while the other groups would appear to be opening; this is an effect of Hubble's law, everything is moving away from any observer at 67.80±0.77 (km/s)/ Mpc, thus the farther away, the faster it is going away no matter where you are . Even at that, I've seen several Hubble images showing galaxies colliding just like we're about to do with Andromeda.

It's possible. The probability that a given star is behind any star is about 10^-15. To get a large portion of the Milky Way stars behind other stars, we're talking about a probability of about 10^-1500000000000.

Is it possible that other stars are just hidden behind other stars and that contributes to a large portion of the missing mass?

No. If hidden by dust, we'd see more infrared heat in space as light emitted by stars has to go someplace. If hidden directly behind other solid objects (besides being so astronomically against the odds that things are only hidden from us), it wouldn't account for observations of galaxies rotations speeds that we see along the axis of rotation rather than against the edge. Even for the galaxies we see on edge, if they were all weighted with the mass on the other side of where we are, the rotations speeds, l

Dark matter is the only real solution left standing at this point and the astronomers and scientists of the world had to be drug to that conclusion, kicking and screaming, over the decades, long before the public started hearing about it.

Maybe if the ones who started talking about it had used a diffrerent term than "dark matter", it would be easier to accept. We hear about quarks, leptons, muons, and things with spin and flavor, etc. I don't understand all that, since I am not a scientist, but I can believe it is serious. Calling it "dark matter" was a dumb move, because it makes it sound as believable as "pixie dust" or "magic beans". At least, they could have used the Japanese words for it like they did with "tsunami".

Actually, just the opposite. I even mention quarks as one of the 'weird' things that they made up a term for. "Black hole" isn't all that unusual of a term, and is quite easy to make a mental image of. Color goes with flavor and spin, which are simply properties of things, and the terms can be used creatively with little worry.

But calling the stuff that we can't see, but know it must exist because the equations say so, as "dark matter" was just stupid. The name causes more confusion than it should, even in

Say you have a large aggregation of mass that is orbiting a large, semistationary singularity-- like, a galaxy does.

Outside this rather bumpy gravity well, you have a diffuse cloud of antimass, which then pushes on, and chases the mass as it rotates around the central mass. This pushing cancels out the centripetal force.

It's an interesting idea, as it was recently postulated that there is no real compelling reason for antimass to not exis

Helioshperes as well as our own Ionosphere (magnetic field) have shown that magnetism is a universal force to be reckoned with.
Indeed gravity could be just a magnetic effect on an atomic scale. The more mass the more effect.
Electrons pulsing in a atom synchronizing with identical atoms become a united energy. That's what holds things together.
And what gravity is, basically. A united atomic force. Too simple?
.

Okay, you've strung words together in an evocative way. (Honestly, I don't know of any way electrons "pulse", or any way atoms synchronize with identical atoms - not to mention that atoms with different numbers of neutrons behave very similarly in many way.) If you can make some sort of mathematical model or theoretical framework that agrees reasonably well with current observations, we can start considering it.

In the meantime, remember that we have ways of detecting magnetic fields, and they aren't st